Because of the physical properties of online process monitoring, i.e. monitoring taking place in real time during the working process, in laser cutting it is often not possible to be able to measure the quality features of the processing result directly from the process emissions. In contrast to laser welding, where some quality features can be visualized directly from the melt pool and the resulting seam by means of camera images, in laser cutting, in particular, the quality of the cut surfaces for a coaxial detector device, e.g. a camera, arranged in the cutting head, are too greatly shortened or not visible at all, as the cut surfaces are perpendicular to the direction of observation. Therefore suitable characteristics must also be found from detector signals or camera images which correlate more or less reliably with the cutting quality features. The extent of the correlation can vary depending on the process state and sheet thickness, as a result of which the online assessment of the cutting quality is always subject to uncertainty.
Almost exclusively in-process or online methods have been published for laser material processing, in particular laser cutting. Hardly any post-process or offline methods of assessing the processing quality have been published. More particularly such post-process methods, which without partial removal immediately after processing, or intermittently with regard to the processing, can assess the component quality or certain features thereof.
For example, DE112012005889T5 discloses a very simple offline method in which test parts are cut at a certain location of the metal sheet before or during laser cutting. With a suitable sensor it is checked whether the test parts have fallen out. If so, the processing is started or continued, if not the processing is stopped. Similar procedures can also be found in other documents.
Thus, DE102011004117A1 discloses a method of monitoring laser-cut workpieces. After cutting a (small) inner contour or hole, the cutting head is positioned in the center in the cut contour at a distance from the cutting edge and through renewed irradiation with the working laser and by means of a simple detector in the inner contour or hole it is checked whether the waste part has fallen out or not. If no emissions occur during irradiation, the hole has fallen out. The actually cutting or piercing procedure is not interrupted.
A similar method is disclosed by DE102010039525A1 according to which checking for the presence of the waste part of the cut inner contour is carried out with the distance sensor of the cutting nozzle, wherein the current distance is compared with a known reference distance if necessary, Both methods are used for the quality control of perforated metal plates before these perforated metal plates undergo further processing stages.
JP2000158169A sets out a laser cutting machine with cutting process monitoring by way of a CCD camera wherein the camera recognizes the shape of remainder of the workpiece. A laser projector scans, along a zig-zag path for example, the entire area of the residual workpiece and the camera records the scanned beam of the entire processing area. The cut contour is not considered.